1. Field of the Invention
This invention relates to an image processing system provided with an image memory, which can print a halftone image.
2. Description of the Prior Art
Heretofore, many kinds of printers have been proposed according to various principles as an output terminal for a personal computer, a workstation or the like. In recent years, especially, monochrome laser beam printers (hereunder abbreviated as LBPs), each of which employs an electrophotographic process and laser techniques and excels in a recording speed and print quality, have rapidly come into wide use.
Meanwhile, there have been upsurged demands for a full color LBP in the market. However, in case of a full color LBP, not only color halftone image data but also binary image data as treated by monochrome LBPs are objects to be outputted. The full color LBP is, therefore, required to process halftone image data, as well as binary image data.
Generally, image output equipments such as LBPs employing an electrophotographic process have a defect in instability of results of the electrophotographic process itself. Thus monochromatic or two-level outputs are often employed in such image output equipments. In conventional monochrome printers, a binary dither method is frequently used for printing a halftone image portion.
Hereinafter, the principles of the dither method will be described by referring to FIG. 39. First, pixels of an image are partitioned into blocks a, each of which is a N.times.M pixel grid (incidentally, in case of this example, a 4 by 4 pixel grid). Then, the intensity or tone level of each pixel of one of the blocks a is compared with the dither value of a corresponding element of a corresponding one of threshold matrices b, each of which is comprised of N.times.M elements (incidentally, in case of this example, a 4.times.4 elements). Further, the intensity level of each pixel thereof is changed to binary level (namely, 0 or 1) according to a result of the comparison. Namely, if the intensity level of a pixel thereof is greater than the dither value of a corresponding element of the corresponding threshold matrix, the intensity level of the pixel is changed to 1. Otherwise, the intensity level of the pixel is changed to 0. Such a processing is repeatedly performed on each of the other blocks. consequently, a dither image c is obtained. Incidentally, there are two types of threshold matrices. One is a threshold matrix of what is called the dot concentration type, which is employed to concentrate dots and obtain smooth tone. The other is a threshold matrix of what is called the dot dispersion type, which is employed to disperse dots in case of making much of resolution.
FIG. 40 illustrates the configuration of a circuit for performing a dither method. When a pixel to be compared is serially selected from pixels of a block of an input image represented by an input image signal, an element of the threshold matrix, which has a row and column numbers thereof corresponding to the position of the selected pixel in the block, is addressed or accessed. Then, the threshold or dither value of the addressed element is read out of a matrix memory. Subsequently, the intensity level of the selected pixel, which is indicated by the input image signal, is compared to the read dither value of the addressed element to obtain binary level of the selected pixel. Thereafter, data representing a two-valued image (hereunder referred to as a binary image) to be printed, which have been obtained by calculating binary levels of pixels of the input image as above described, are sent to a printer engine and then are printed. The binary dither method described above, however, has defects in that dither patterns are noticeable in the printed image and that the resolution in a halftone portion is low.
Recently, has been developed a printer engine which can print a multi-level image having more than two intensity levels. Even a system employing such a printer engine, however, has drawbacks in that a memory, the storage capacity of which is 30 megabytes (MB) or more, is needed for printing a full-color image on paper, the size of which is A4, by using the resolution of 300 dots per inch (dpi) and 256 intensity levels, in that the costs of a printer become high and moreover the size thereof becomes large, in that when employing a low-cost two-level (namely, monochromatic) printer, picture quality is low and in that when employing a multi-level printer capable of printing a full color image, a bulk image memory is required and thus the cost becomes considerably high. The present invention is created to eliminate the above described drawbacks of the prior art.